Detectors of the above type generally comprise a high vacuum envelope connected with a vacuum pump to bring the envelope to a pressure sensibly lower than ambient pressure. At least a portion of the envelope is separated from the external environment through a wall or membrane of a material selectively permeable to a test gas, such as for instance quartz or glass with high silica content (>80%).
The membrane may be implemented as a capillary tube or planar or arched window.
An electrical resistor located within the membrane and connected to an electric power supply allows for bringing the membrane to a temperature of at least 300° C., where the glass with high silica content is permeable only to helium.
An electronic control unit of the vacuum pump is capable of detecting variations in the electrical current absorbed by the pump and consequently of signalling the test gas diffusion into the vacuum envelope and hence the existence of a leak.
Such a detector is disclosed for instance in patent EP 352,371 in the name of the Applicant.
According to the prior art, the leak detector further comprises a sampling line for the intake of a gas coming from a probe or a sniffer. This gas is directed to a membrane to come to a contact therewith. The gas exposed to the membrane is directed to a discharge line to be exhausted to the outside. The sampling line also comprises a suction pump to suck a greater gas amount through the probe. The pump is generally located between the probe and the membrane.
Leak detectors are generally used to check the tightness of containers, reservoirs, packages etc., often in a quick sequence, such as, for instance, during production and packaging industrial processes.
Consequently, the leak detector has to rapidly recover the initial operating conditions, immediately after the detection of a leak, so as to be ready for a further measurement.
One of the problems to be solved in leak detectors utilising in manufacturing processes is thus how to quickly purge the feeding and discharge lines so as to remove any residual test gas possibly present in the detector before starting a further test.
A known method of purging the leak detector comprises the step of passing clean air. without test gas, through the detector. Such operation can be obtained for instance by moving the detector or the probe, or sniffer from the test zone to a neutral zone where clean air is present.
That method is however complex and considerably time consuming; moreover, it can be applied only to portable leak detectors, whereas it cannot be used in case of detectors that are located along a working line.
It is a main object of the present invention to overcome the above drawback by providing a leak detector and a method of its operation, which allows for effective purging of the sampling and discharge line.
Another problem in prior art leak detectors is the risk of saturation because of very high concentrations of test gas inside the vacuum envelope. Actually, an excessive amount of test gas inside the envelope requires a long time for being evacuated by the vacuum pump and hence delays the reset of the operating condition in order to start a further test.
It is another object of the present invention to provide a leak detector comprising a purging system that allows for avoiding excessive test gas concentrations in the detector.
The above and other objectives achieved by a leak detector and method of its operation as claimed in the appended claims.